This invention relates to the measurement of radiation and, more particularly, to means for determining the amount of radiation received by personnel.
It is a requirement that personnel who may be exposed to harmful radiation be provided with some means for determining the amount of radiation they may have been exposed to. These personnel include those working in the health care field (where exposure to diagnostic x-rays is a hazard) as well as those handling nuclear or radioactive materials. A fairly rigid system exists for monitoring the radiation exposure of such personnel. Typically this procedure involves the issuance of dosimeter badges which measure the amount of radiation to a person has been exposed over a period of time. These badges are periodically checked to make sure that the person wearing the badge has not received more radiation than is medically safe.
However, the rigid process set up for full time employees is not readily applicable to temporary employees and visitors. This is particularly true for visitors and patients to health care facilities, as well as visitors and contractor-employees visiting nuclear facilities. The same is true of research laboratories that utilize radioactive materials. Here, the difficulty arises with respect to visiting staff.
With the detection technique utilized with the permanent staff of a facility, the dosimeter badges are issued on a periodic basis and then collected. The amount of radiation is then measured by rather sophisticated equipment, e.g. expensive, battery powered, hand-held readers. Since the temporary personnel will only be at the facility for a short time, there is a need for means to measure the amount of radioactive dose quickly and without undue complication, so that it may be performed by ordinary personnel who have not been particularly trained in this area.
The major drawbacks of film and thermoluminescent detector ("TLD") badge systems are (1) nonlinear low energy response, (2) the loss of information upon reading and (3) the requirement for a measuring system that is too formal for nonregistered workers and visitors.
In recent years there has been an increased use of electret ionization chambers to detect radiation. These chambers employ an electret, which is a material that may receive and hold an electrical charge. It maintains this charge over a relatively long period of time without reduction, except in the presence of radiation. When it is exposed to radiation, the radiation causes a reduction in the charge which is directly related to the amount of radiation the electret is exposed to. An explanation of the theory of charge reduction in electret ionization chambers due to x-rays and gamma-rays is set forth in an article by Pretzch et al., Vol. 4, p. 79, Radiation Potential Dosimeter (1983). Many designs of electret ionization chambers have been proposed for personnel radiation dosimetry. Examples are contained in the articles by Bauser et al., Health Physics. Vol. 34, p. 97 (1978); Cameron et al., Proceedings on the Sixth Conference on Dosimetry (1980) and Ikeya et al., Health Physics, Vol. 39, p. 797 (1980). However, these devices have not yet gained the confidence of end users because they generally lack a sufficiently accurate electronic measuring system to read and record the cumulative amount of radiation dose.
Electret ionization chamber devices do not suffer from the same problems as film and TLD badges. However, it is difficult to use conventional, easy to operate equipment to measure charge reduction in electret ionization chambers without destroying the charge.
The standard methods of measuring the surface potential of electrets require access to the film surface. These are generally unsatisfactory since they do not allow repeated measurement. Further, there are sonic methods of measuring electret charge as set forth in the above described article by Ikeya et al. These sonic methods have inaccuracies associated with dimensional variations and the effects of the environment. According to the sonic method the electret charge is measured by detecting mechanical oscillation. The mechanical oscillation of the electret creates an A.C. signal whose magnitude is proportional to the charge and the amplitude of the mechanical motion. Thus, the A.C. response is an indication of the electret charge. However, it is sometimes difficult to measure the true amplitude of the A.C. signal, and to calculate the effect of changes in mechanical oscillation.
Therefore, it would be advantageous if a personnel radiation dosimeter could be developed which would provide not only accurate readings of the accumulated dose, but which could also be read with a rather simple device.